Hi Sameek,
To answer your questions:
1. Here is the documentation for security within mendix. It explains the different levels of security pretty well
https://docs.mendix.com/refguide/security
2. You can take an object from one page and pass it to another. In this case you would have an object that had an attribute that held the query string, and you would pass that object from page 1 to page 2
Additional to the answer of Austin;
Modeling in Mendix is comparable to Object Oriented Programming. Java objects can be passed through the application using the out of the box functionalities in pages & microflows (custom logic)
Learning paths can be found here => http://learn.mendix.com
The Rapid developer learning path is a good starting point